Existing soldering iron technology employs a “wettable” soldering tip. In other words, the soldering tip is designed to facilitate the spreading of molten solder over its working surface. Metals such as iron, iron-nickel, and other iron alloys are often used in soldering tips since they are wetted by molten solder and yet do not deteriorate extensively while in contact with the molten solder.
Wetting may be empirically observed by placing a droplet or bead of a liquid on a surface. The shape of the bead qualitatively indicates the nature of the wetting between the liquid and the surface. If the bead flattens out much like a pancake, then the liquid wets the surface. If the bead remains nearly spherically shaped, like water on a recently waxed car surface, the liquid does not wet the surface. Thus, a molten solder that does not wet a surface of a soldering tip will, under the influence of gravity or some other force, tend to be shed from that surface. In contrast, a molten solder that wets the surface of the soldering tip will tend to cling or adhere to that surface. As previously mentioned, molten solders tend to wet iron and iron-nickel alloy soldering tips.
Quantitatively, a contact angle formed between a droplet of molten solder and the surface of the soldering tip may be measured to determine the wettability of the molten solder on the soldering tip. The contact angle is measured between the interface of the molten solder and a surface of the soldering tip and the surface of the droplet of molten solder. Thus, contact angles measured greater than 90 degrees indicate that the molten solder does not wet the soldering tip. That is, the droplet of molten solder is somewhat spherical. On the other hand, contact angles less than 90 degrees indicate that the molten solder wets the surface of the soldering tip. In other words, the droplet of molten solder flattens and tends to adhere or cling to the surface of the soldering tip. Therefore, while soldering with a wettable soldering tip, molten solder adheres to the soldering tip.
One problem with existing soldering tips is the variability in the amount of solder transferred to the joint. In particular, this problem manifests itself in high-volume soldering processes, such as soldering electrical connections in electronic devices and the like. Although the amount of solder transferred depends upon a number of factors, one primary factor is the amount of the solder that clings or adheres to the soldering tip. In other words, the amount of solder transferred to the joint is directly related to how much solder adheres to the tip from one joint to the next.
Not only is the variation in the amount of solder transferred to the joint a problem, but it is difficult or impossible for an operator to determine the amount of heat transferred from the soldering tip to the joint. Heat transfer to the workpieces depends on the amount of molten solder between the workpieces and the soldering tip. Thus, variable amounts of molten solder on the soldering tip cause variation in the quantity of heat transferred to the workpiece. In many instances too much heat will damage the component, and, on the other hand, too little heat or too little solder, or both too little heat and too little solder, may result in a poorly formed joint. If that is not enough, a host of other problems ensue due to the wettable nature of the prior art soldering tips. For instance, solder spikes and solder bridges between successive connections are known to form due to the molten solder adhering to the soldering tip as the tip is withdrawn.
Accordingly, a soldering system having a soldering tip that facilitates a repeatable transfer of solder to a series of successive joints is needed.